"Why is the Senate called the upper house and the House of Representatives called the lower house?"

Thanks for your question. The House of Representatives—sometimes referred to as the lower house—and the Senate—sometimes referred to as the upper house—are part of the Australian Parliament. The reference to upper and lower house is a tradition the Australian Parliament borrowed from the British or 'Westminster' Parliament. The British House of Commons is often referred to as the lower house, and the House of Lords is often referred to as the upper house.